Bleach isn’t toxic to the environment per se: it’s very dilutely in most municipalities’ drinking water.
But putting a 10% bleach solution on your horse’s feet is not healthy for the living tissue.
People forget that the area that gets thrush, you know, the frog, is non-keratinized tissue. It’s more similar to you or your horse’s skin than it is to the rest of the hoof. When it gets an infection (thrush), it is not much different than an infection on your own skin.
Anyone who handles dilute bleach solutions a lot with bare hands (guilty) knows that if you get a little on you every now and then, there is no serious damage. Your skin looks a little lighter and feels dry, but then sloughs off. But if you are getting it on your hands on a regular basis, it continues to make your hands drier and drier. They may even crack and bleed. If you had a sore or wound on your hand, it wouldn’t heal well-- conversely, the constant bleach exposure would continue to dry out the delicate, healing tissue and make the wound close more slowly.
The same thing happens to your horse’s frog. Not to mention the fact that too much bleach could dry out the solid hoof, just like it does the tissue, causing more negative side effects.
A little bit of anything is not harmful once in a blue moon. Sure, the bleach will help clean the hoof and dessicate any minor funky areas if you use it sparingly. But if you’re using it on any sort of regular interval, it’s going to do more harm than good in the long run. Especially if you have an active infection in the area.